Naturally occurring and synthetic zeolites have been demonstrated to exhibit catalytic properties for various types of hydrocarbon conversions. Zeolites, which are ordered porous crystalline aluminosilicates, have definite crystalline structure as determined by X-ray diffraction studies. Such zeolites have pores of uniform size which are uniquely determined by unit structure of the crystal. The zeolites are referred to as "molecular sieves" because interconnecting channel systems created by pores of uniform pore size comparable to those of many organic molecular cross sections, allow a zeolite to selectively absorb molecules of certain dimensions and shapes. The pores systems in porous zeolites may be categorized as small, medium or large pore size, depending on the number of oxygen atoms in the ring systems which define the apertures to the interior pore structure of the zeolite. See Shape Selective Catalysis in Industrial Applications, Chen et al, Marcel Dekker, N.Y. 1989, ISBN 0-8247-7856-1.
The most important groups of zeolites used industrially for catalytic and other applications such as sorption are the medium (intermediate) and large pore size zeolites. Examples of the former include the widely used zeolite ZSM-5 as well as other materials such as ZSM-23 and ZSM-35. These zeolites are widely used in petroleum refining processes (catalytic dewaxing, FCC additive catalyst) as well as in petrochemical processes (ethylbenzene production, xylene isomerization), to name but a few examples. The large pore zeolites which enjoy the greatest commercial use are the faujasite zeolites Y and ultrastable Y (USY); these are widely used in petroleum refining processes such as FCC and hydrocracking.
Compositionally, zeolites are metallosilicates, with the aluminosilicates being the normal natural form for the zeolites which are found in nature, although other metallosilicates such as borosilicates and ferrosilicates have also been described. In addition, the ratio of silicon to metal in a zeolite may vary from relatively low values to very high ones, extending in principle to infinity, so that the ultimate material is a polymorph of silica. See, for example, "When is a Zeolite not a Zeolite", L. V. C. Rees, Nature, 296, 491-2, Apr. 8, 1982. For brevity, zeolites will for the most part be described here as aluminosilicates although it should be remembered that other metals besides aluminum may replace all or part of t he aluminum content of a zeolite. In terms of an empirical formula , zeolites m ay be defined by the formula: EQU M.sub.2/n O.sub.x Al.sub.2 O.sub.3x (SiO2).sub.y. H2O
In the empirical formula, x is equal to or greater than 2, since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, and n is the valence of the cation designated M. See, for example, D. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York p. 5 (1974). In the empirical formula, the ratio of the total of silicon and aluminum atoms to oxygen atoms is 1:2. M was described a s sodium, potassium, magnesium, calcium, strontium and/ or barium, which complete the electrovalence makeup of the zeolite.
The structural framework of a zeolite is based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygen atoms so that the ratio of the total aluminum and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing alumina is balanced by the inclusion in the crystal of the cation, for example an alkali metal, an alkaline earth metal or an organic cation. This can be expressed in the formula above where the ratio of aluminum to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged entirely or partially with another type of cation utilizing ion exchange techniques which have now become conventional. By means of such cation exchange, it is possible to vary the properties of a given aluminosilicate by suitable selection of the cation. In the as-synthesized materials, the cavities and pores are occupied by molecules of water prior to dehydration and/or possibly by organic species from the synthesis mixture.
As previously mentioned, the silica/alumina atomic ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with silica/alumina atomic ratios of from 1.5:1 up to 3:1, while that ratio in zeolite Y is from 3:1 to 6:1. In the synthetic zeolite Ultrastable Y (USY), which is made from zeolite Y by a process of successive ammonium exchange and steaming, the silica:alumina ratio can be made to exceed the value of 6:1 typical for zeolite Y and extend up to high values indeed. In some zeolites, the upper limit of the silica/alumina atomic ratio is unbounded. ZSM-5 is one such example wherein the silica/alumina ratio may extend up to infinity. U.S. Pat. No. 3,941,871 (RE. 29,948),discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added aluminum and ex hi biting the X-ray diffraction pattern characteristic of ZSM-5 zeolites.
The silica/alumina ratio of the "as-synthesized" zeolite can be increased by decreasing the tetrahedral alumina content of the zeolite. Decrease in the tetrahedral alumina may be effected by synthetic methods developed to deplete the tetrahedral alumina of a zeolite. In addition, the silica:alumina ratio of a zeolite may be increased (loss of tetrahedral framework alumina) as a result of process conditions to which the zeolite is subjected during use. Process conditions which will effect depletion of tetrahedral alumina include high temperature calcination and steaming.
Increased silica:alumina ratio in zeolites is associated with increased stability to hydrothermal degradation: zeolites with relatively high silica: alumina ratios are more resistant to the effects of steaming in that they retain crystallinity and catalytic activity buffer than zeolites of lower silica:alumina ratio. It has therefore been considered desirable to use zeolites of higher silica:alumina ratio in many applications where hydrothermal conditions are encountered either during the direct use of the zeolite or when the zeolite catalyst is undergoing regeneration. One application of this type is in the fluid catalytic cracking (FCC) process where the zeolitic catalyst is exposed to high temperatures and copious quantitites of steam during the regeneration step when the coke which accummulates on the catalyst is oxidatively removed prior to recycle of the catalyst to the cracking step. Historically, the FCC process which initially used zeolitic catalysts based on zeolite X (silica:alumina ratio up to 3:1), progressed initially to the use of catalysts based on zeolite Y (ratio of 3:1 to 6:1) and finally to zeolite USY with ratios of 6:1 or higher. The use of zeolite USY has resulted in both process improvement in terms of catalyst stability as well as in a more desirable slate of products and product properties. Zeolite USY is now used in a number of other catalytic applications requiring a large pore size zeolite, for example, hydrocracking.
Various treatments have been proposed for modifying the physical and chemical properties of zeolites. An important method in reducing the activity of zeolite catalysts is by the process of steaming. By controlled steaming, it is possible to produce zeolite catalysts having any desired degree of activity: The degree of steaming of a specified catalyst to achieve a desired activity level is largely dependent upon the nature of the zeolite. Steam treatment, however, often requires long periods of time to treat the catalyst effectively for activity reduction.
U.S. Pat. No. 3,939,058 discloses methods of modifying the catalytic properties of zeolites. One such method is calcination which is defined as heating at high temperatures but below the sintering temperature of the zeolite for varying periods of time. Other methods are also disclosed, including compositing the zeolite in a matrix and steam treatment. The patent further states that the crystallinity retention of catalysts may be improved by precalcination of the crystalline aluminosilicate. For example, the patent states that it has been found possible to preserve the crystallinity of aluminosilicates such as the rare earth exchanged synthetic faujasites, by calcining the zeolite to drive off water, thus forming a more suitable structure and minimizing loss in crystallinity during subsequent rapid drying, as in spray drying, wet processing, steaming and aging. The calcining may be accomplished by heating the crystalline aluminosilicate sieve after ion exchange to a temperature below the sintering temperature of the sieve and generally in the range of 260 to 870.degree. C.
Similarly, U.S. Pat. No. 4,141,859 discloses a method of controlling the relative acid activity of zeolite catalysts, by treating the zeolitic component with air or steam at elevated temperatures, e.g., up to 925.degree. C. in air.
Calcination of the freshly synthesized zeolite to remove adsorbed water and any organic materials that have been used to form the zeolite crystals is necessary to activate the zeolite and accordingly has generally been employed. Also, as stated above, precalcination of the zeolite has been found to stabilize the crystallinity of the zeolite. However, heat treatment may remove hydroxyl groups from the framework of the zeolite. Thus, dehydroxylation of a decationized Y zeolite is discussed in Zeolite Chemistry and Catalysts, ACS Monograph 171, pages 142 and 143, in which dehydroxylation of Y zeolite is stated to result from prolonged calcination at relatively high temperatures, resulting finally in the structural collapse of the zeolite and the formation of an amorphous silica or silica-alumina structure. For these reasons, the use of high temperatures has generally been avoided in zeolite synthesis. When organic materials are to be removed from the freshly synthesized zeolite, temperatures of about 540.degree. C. are typical and generally not exceeded in order to avoid damage to the crystal structure.
Calcination or high temperature treatment has been employed in various catalyst treatments to achieve particular results, for example, to convert impregnated metal or other compounds to different forms as described in U.S. Pat. Nos. 4,276,438 and 4,060,568 or to destroy ion exchange capacity as described in U.S. Pat. No. 3,097,115. However, even in such cases the use of higher temperatures, e.g. above 500.degree. C., has not been preferred because of the undesirable effect on the structure of the zeolite.
Other high temperature treament processes applied to zeolites are described in U.S. Pat. Nos. 5,143,876; 5,102,839; 4,783,571 and 4,141,859. U.S. Pat. No. 5,227,352 describes a method for producing crystalline aluminosilicates by the termal shock treatment of zeolite USY; according to the description of the method it is essential to use USY as the starting material rather than zeolite Y itself.
Besides the specific pore configuration of a zeolite, another indicium of its selectivity is the zeolitic surface area (ZSA) and its relationship to the mesopore area (MSA). Shape selective reactions take place at the active sites in the zeolite created by the presence of the trivalent metal atoms in the zeolite structure; reactions which are not constrained by the pore structure of the zeolite--the non shape selective reactions--may occur at catalytically active sites in the larger pores of the mesopore regions of the zeolite. The acidic catalytic activity of the zeolitic tetrahedral sites is also greater than the activity of similar but non-zeolitic sites. So, if the ZSA is relatively large compared to the MSA, shape selective reactions will be favored as compared to the non-shape selective reactions both by reason of the relatively greater zeolitic surface area available for the shape selective reactions and by the relatively greater catalytic activity of the zeolitic sites. For this reason, a high ratio of ZSA to MSA is preferred. So far, no treatments specifically designed to modify the ZSA and MSA of a zeolite have been described.